The present invention relates to a digital image forming apparatus. More particularly, the present invention relates to a digital image forming apparatus having a functionality of saddle stitching, in which a plurality of prints generated by image forming are stacked and stitched at a center of a spread page as in the case of weekly magazines.
The term “spread page” refers to the innermost sheet of saddle stitch type prints when they are gathered, and the term “saddle stitch type prints” refer to printed paper sheets that are produced for saddle stitching but not stitched yet.
A conventional copy machine having this type of saddle stitching functionality is disclosed in U.S. Pat. No. 5,897,251. With this copy machine, in order to allocate images of e.g. the third and fourth pages to a spread page of saddle stitch type prints, printing is conducted with fourth page replaced with the fifth page or with a white paper sheet inserted before the third page.
Also, the conventional copy machine is not automatically capable of arranging an image (e.g. a full-color image) of a specified image document among a plurality of documents on a spread page, that is, the innermost page of the saddle stitch type prints in gathering. To obtain such prints with the conventional copy machine, an operator first needs to select from the documents two pages to be arranged on the front side of the spread page and another two pages to be arranged on the back side of the spread page. These four pages are then color-copied in the saddle stitching mode while the remaining pages are copied in monochrome in the saddle stitching mode. Finally, these two sets of prints are combined and bound at the center of the spread page to gather saddle stitch type prints. Thus, the saddle stitch type prints with the conventional copy machine requires an operator to carry out bothersome and complicated operations because two kinds of prints obtained by two separate copying processes should be combined.